The present invention relates generally to an improved resistance weld-based gun assembly for use in an electric welding system, and more particularly, to an assembly selectively including a resistance-based force control or a mechanical non-electrical closed loop coolant system to eliminate the need for plant-provided facilities.
Resistance welding utilizes the flow of electricity to permanently join two or more overlapping metallic workpieces to one another. Typically, the metallic workpieces are placed between two opposing electrodes of an electric welding system gun assembly. The electrodes are then forced together until their tips contact the outer surfaces of the workpieces at a pressure sufficient to sandwich the workpieces and ensure an adequate electrical contact between the electrodes and the workpieces. Then an electrical current is induced to flow from one electrode tip to the other electrode tip by way of the sandwiched workpieces. The workpieces act as conductors in the resulting electrical circuit, and resistance to the flow of electrical current at the interfaces between the metals generates heat. The affected metal of each workpiece selectively becomes molten, and interacts with molten metal of an adjacent workpiece to form a weld nugget that permanently bonds the workpieces together at the point of electrode tip contact.
A number of factors relate to the creation of a weld nugget, including the force and area of contact between the electrode tips and workpieces, the level of current flow, the length of time that the current flow lasts, degree of workpiece imperfection, and even the condition of the electrode tips themselves.
The prior art teaches the importance of creating an adequate weld nugget. Therefore, weld systems are over-configured to generate a weld nugget even if there are significant workpiece imperfections by having high force, current levels, and current application times. Yet, many resulting welds are still imperfect. Therefore, typically, 25% of all welds in a part are added to insure adequate structural integrity.
Further, such overcompensation for possible workpiece imperfection results in significantly higher deformation (mushrooming) of the electrode tips at the point of contact between the tips and the mating workpieces. If the electrode tips are inadequately cooled, the electrodes experience excessive tip wear, deformation, tip sticking and even tip melting, all of which contribute to poor weld quality and increased maintenance. The generation of significant heat at the electrode tips also results in significant heat built up in the welding control unit, transformer, and secondary (i.e., high current) cable disposed between the electrodes and the transformer.
Moreover, the application of continuous significant electrode force upon the sandwiched workpieces requires the use of significant sources of compressed air. The compressed air provides for the actuation of various air cylinders to position the welding gun electrodes with respect to the workpieces to be sandwiched therebetween and to generate force.
The use of complex air and water systems with their multitude of hoses and corresponding pipes, valves, and the like, in combination with the controllers and supply mechanisms, greatly increases manufacturing expense. It has been estimated that approximately 25% of the total cost of an electric weld system can be attributed to the use of external air and water-cooling circulation systems.
Typical electrical welding systems must be custom designed, built, and tested, requiring the services of numerous skilled trades. Following such testing by the supplier, the verified welding systems are then tom-down, transported, and rebuilt at the final manufacturing facility. Such intermediate steps significantly increase the time lag in providing a complete electric weld system. Moreover, both the design and testing facility, as well as the final manufacturing facility, must make significant capital and continuous investments in air and water-cooling circulation systems.
Nor is the problem limited to manufacturing of an electric weld system itself. The ongoing maintenance problems of requiring significant water-cooling and air circulation systems are extensive. It has been estimated that possibly over 80% of the down time of a typical electric welding system may be attributed to the host of air hoses, and feed and return cooling water hoses in combination with the corresponding pipes, valves, and the like.
There are additional costs to requiring complex water and air supply circulation systems. Each electric welding system becomes unique. Each length of hose, each bend in a pipe or conduit, and each selected placement for various cooling water fittings is necessarily tailored to the particular welding system. The kinematics of the host of hoses (pejoratively referred to as xe2x80x9cspaghettixe2x80x9d) cannot be accurately predicted or modeled. Accordingly, the robot movements in each work cell must be inputted on-site, step-by-step, to ensure that hoses do not become entangled. To further exacerbate the problem, the resulting xe2x80x9cwindowxe2x80x9d in which a robot arm may move to reach, for example, a weld point, is significantly reduced, again due to the proliferation of the compressed air and water hoses and associated components. Thus, the time to program a robot arm is extensive and the resulting process time to process workpieces is significantly increased.
In a manufacturing plant having a large number of electric welding systems, the aggregate cost in having to individually construct, install, and maintain each electric weld system is extensive. Accordingly, there is a need to provide an improved electric welding system that minimizes or eliminates one or more of the problems as set forth above.
The present invention is directed to an improved weld gun assembly for use in an electric welding system that includes various features to eliminate the need for external plant-based external air and water-cooling circulation systems with their attendant complexity, expense, and maintenance issues.
A first aspect of the invention includes a weld gun having opposing electrodes continuously separable through a predetermined range for providing welding current through at least two metallic workpieces. The weld gun includes a welding secondary circuit, including transformer secondary windings, in combination with a transformer and weld control, and an electrically actuated, electronically-controlled actuator operatively coupled to the weld gun. The actuator is capable of moving the electrodes to any one of a plurality of electrode separation distances, the electrodes adapted to contact the workpiece at carefully controlled force levels. Preferably, the electrodes contact a workpiece softly (i.e., with accurately controlled minimized force). For low work cycle welding, without cooling, no further features are required. For higher work cycles, a resistance-based force controller works in combination with a host control coupled to the actuator for controlling the actuator in accordance with predetermined criteria including a signal generated by the resistance-based force controller.
In operation, the actuator preferably includes a calibration step, wherein it is reset to zero with the opposing electrodes touching one another at a predetermined force level. At this point, the closed position secondary resistance value can be used to determine the condition of the electrodes. If necessary, a request for tip dress can be generated.
Preferably, the welding secondary circuit provides a highly conductive current path when the electrodes selectively engage the workpieces, wherein the workpieces are less conductive than the secondary circuit. Further, the resistance-based force controller preferably includes at least four leads, an excitation lead typically going to each of the electrode arms and a measurement lead also going to each of the electrode arms. In operation, the resistance-based force controller selectively excites the secondary circuit when the electrodes are not in contact with a workpiece, by applying a current through said secondary circuit by means of each excitation lead, and selectively measures a resistance of the secondary circuit by means of each measurement lead.
When a workpiece is about to be welded the force controller excites the secondary circuit and measures the total resistance of the secondary circuit and the resultant resistance of the workpiece conduit path after the actuator applies a predetermined force upon the workpiece. A calculated resistance of the workpiece conduit path is compared to predetermined criteria. If the resistance is not within the predetermined criteria, as for example, when a work piece has unexpected imperfections, the force controller selectively generates a signal to the actuator to adjust the predetermined force applied by the actuator to the workpiece by a predetermined increment. The iterative process continues until the predetermined criteria are met and the weld gun assembly then generates one or more welds. Alternatively, the force controller may continuously monitor and measure resistance (voltage drop) while the gun control continuously increases force. The force controller may interrupt the gun control when the appropriate value is achieved to generate a weld.
Significant advantages result. First, resistance value detection and control with dynamic optimization of weld parameters such as force, current, voltage and electrode displacement helps ensure consistent good weld nuggets. Thus, the number of weld nuggets needed for a particular part significantly decreases. Second, the use of resistance value detection to ensure that an adequate electrical circuit has been made between the two electrodes and the workpieces permits the use of a much lower electrode force application upon the workpieces. Such a lower force minimizes gun deterioration such as damage to the electrode tips, and gun mechanism. Third, expulsion, with its undesired loss of molten metal preferably used to create a weld nugget, is reduced. Fourth, having precise and infinite opening positions for the electrodes reduces cycle time. Fifth, electrode deformation is reduced, allowing higher current density with equal current. These and other advantages combine to allow successful welding at generated temperatures without the need for external facility based water-cooling.
Thus, the use of non-facility based isolated cooling mechanisms become possible even in xe2x80x9chigh work cyclexe2x80x9d processes. One such mechanism comprises a reciprocating coolant heat extractor. The extractor includes a highly conductive coolant, a cylinder with an actuator such as a mechanically operated piston, a heat exchanger, and a closed loop system for the coolant that includes the actuator and the heat exchanger. In operation a piston moves in the cylinder upon operation of the weld gun assembly to circulate the coolant so that it picks up heat from a heat source and releases the heat through the heat exchanger.
Preferably, the piston is mechanically linked to the weld gun. A pivot point is created between the opposing and electrodes of the weld gun, and a mechanical linkage is then secured to the piston and the weld gun. A first end of the mechanical linkage is secured to a pivoting electrode and a second end is secured to the piston. The intermediate portion is secured to the pivot point between the electrode arms. The line containing the coolant includes various pathways and check valves to control the movement of the coolant as it removes heat from hot components and dissipates that heat.
A second non-plant based isolated cooling mechanism includes a bladder device. A check valve is disposed between a source of heat and a heat exchanger and a second check valve is disposed between the heat source and the bladder. As the coolant expands with the addition of heat, it flows from a heat source through the first check valve to the heat exchanger, the bladder expanding to accommodate the increased volume from the expanding coolant. The first check valve selectively closes upon termination of coolant expansion while the second check valve selectively opens to re-supply coolant under force from the bladder to the heat source.
A third non-plant based isolated cooling mechanism relies solely on the natural conduction and convection associated with the electrode arms themselves. Both the mass and the outer surface area of each arm is maximized in a region adjacent a receptacle adapted to receive an electrode so that the heat may be conducted away from the electrodes to an outer surface and dissipated through convection. To increase the surface area of the arms, a series of ribs or undulations having channels defined therebetween or openings within the arms themselves may be used.